It is estimated that some 800,000 health care workers accidentally receive stab wounds each year from hypodermic syringe needles used in their work. This has been of increasing concern because of the spread of deadly infections which is traceable in part to these accidents.
Prior art has employed various types of needle shields but problems have arisen when the shield is to be replaced after partial or full injection of a syringe's contents. The return of the needle to the sheath requires good depth perception because of the relatively small angle of acceptance of the sheath. If an error is made, there is a danger that the technician can thrust the needle into the hand holding the sheath. The prior art of Heydrich (U.S. Pat. No. 2,854,976) and Smith (U.S. Pat. No. 4,643,722) treats this problem by sheath constructions which fit over a needle through a side opening. The sheathing motion is thus perpendicular to the penetrating motion; the likelihood of accidental puncture is thus reduced. In some cases, such as the injection of anesthetics in dentistry, however, the syringe is applied at a number of points. The inconvenience of finding and applying the sheath after each injection discourages its use.
Other problems with prior needle protection devices are with their sterilizability. Close fitting sheaths tend to seal off the needle from the entrance of liquids and gases. The original cleanliness of the needle is thus guarded but a problem develops if the technician attempts to resterilize the sheath.